Preclinical evidence suggests that metformin, a widely prescribed anti-diabetic drug, may inhibit lung cancer progression. We investigated whether metformin use was associated with decreased risk of cancer-specific mortality in lung cancer patients. This study included newly diagnosed lung cancer patients (identified from English National Cancer Data Repository, 1998–2009) with type 2 diabetes (based on UK Clinical Practice Research Datalink prescriptions and diagnosis records). Lung cancer deaths occurring up to 2012 were identified using Office of National Statistics mortality data and the association between metformin use (before and after diagnosis) and risk of lung cancer-specific mortality was calculated using Cox regression models. In analysis of 533 patients, we found a weak non-significant reduction in lung cancer-specific mortality with metformin use after diagnosis (adjusted HR, 0.86; 95% CI, 0.68–1.09). No association was evident for metformin use before diagnosis and cancer-specific mortality in analysis of 1350 patients (adjusted HR, 0.97; 95% CI, 0.86, 1.11). Associations were similar by duration of use. In addition, after adjustment for potential confounders, there was little evidence of an association between the use of other anti-diabetic medications (either before or after diagnosis) and lung cancer-specific mortality; including sulfonylureas, insulin or other anti-diabetic medications (such as thiazolidinediones). Overall, the results from this population-based study provide little evidence of a protective association between metformin use and cancer mortality in lung cancer patients.